I'm trying to power a MSP430 microcontroller at 3.0V. I have a voltage regulator whose output is 3.6V, but I have a Bluetooth module that can only tolerate 3.0V. I plan to use a voltage divider to convert the 3.6V output of voltage regulator to 3.0V. Are there any recommended values for resistors? Will impedance mismatch be a problem?
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12\$\begingroup\$ Never use a voltage divider to power a circuit! Instead, find a voltage regulator whose output is 3V. If you can't then just use a variable regulator like the LM317 to regulate the voltage to 3.0V. \$\endgroup\$– SaadCommented Dec 4, 2012 at 11:51
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6\$\begingroup\$ And if you must, use a diode to drop the 3.6V down to ~3V. \$\endgroup\$– Christoph BCommented Dec 4, 2012 at 12:00
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\$\begingroup\$ Zener diode would work well. \$\endgroup\$– capcomCommented Jan 15, 2013 at 21:02
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4 Answers
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I've had great success with Microchip's MCP1700 line of regulators. Very wide range of output voltages, only 2 capacitors are basically needed. SOT23 package (and even TO-92) is easy to use. Max current is 200 to 250mA which is plenty for MSP430 plus circuitry. And, it's inexpensive. Still, it has short circuit and thermal protection.
Do not use voltage dividers. You'll waste too much power.
MCP1700 Datasheet http://ww1.microchip.com/downloads/en/DeviceDoc/21826b.pdf
answered Dec 4, 2012 at 17:45
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MSP430 is happy to operate at 3.6V. You don't need to drop the voltage at all.
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2\$\begingroup\$ Yes, but I'm using Panasonic's Bluetooth module with it which can tolerate a maximum of 3 V(approx). So I can't operate msp430 at 3.6 V. \$\endgroup\$ Commented Dec 5, 2012 at 11:08
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Well, you could use such a divider. It's just a matter of order of magnitude of the allowed voltage ripple you can afford.
A MCU such as your MSP430 does have variation in its power consumption depending on what it is doing. Theses current spikes generates, due to the power supply line impedance (inductance), voltage drops that are not recommended and you have to put decoupling capacitors close to the MCU pin to reduce the impedance seen by the MCU pin. If you use a voltage divider, you adds deliberately a resistor in the power path on the MCU. Variations of power consumption (A) will leads to voltage drops in the power supply pin. Of course if your voltage divider uses high power resistors or ~1 or 2 ohms (!), the impedance increase on the MCU power pin is negligible but the current drawn by your voltage divider will be of several amps ! I suggest that you use a voltage regulator that is able to provide the right voltage. But if you can't, as Jonny B Good said, add a diode in place of your voltage divider.
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If you're already using a voltage regulator (I assume it is a linear one) then just replace it with adjustable regulator, I'm sure you can find a pin-to-pin replacement to you regulator. And simply set the output to 3.0V with the resistor. If this is somehow impossible for you — use the diodes to drop the voltage, as Jonny B Good suggested.
answered Dec 4, 2012 at 18:37